In order to define the basic properties of microglia within the basal ganglia (BG) of the adult CNS, we used CX3CR1-EGFP transgenic mice to visualize microglia within the ventral tegmental area (VTA), nucleus accumbens (NAc), substantia nigra pars compacta (SNc), and substantia nigra pars reticulata (SNr). Previously we found that microglia populate the VTA at significantly lower density and exhibit sparse branching compared to other BG regions. In contrast, microglia within the SNr are present at a dramatically high density and both SNr and NAc microglia display highly-ramified morphologies. In the past year, we have expanded these analyses to show that there is no clear correlation between BG microglial density and neuronal density or OPC density but there is a consistent ratio of microglia to astrocytes across all analyzed BG nuclei. We also used immunostaining for VGlut1, 2, and 3 to determine if microglial process branching complexity is related to the number of excitatory synapses within the surrounding tissue. This analysis revealed that there was not a consistent correlation between density of glutamatergic synapses and microglial tissue coverage. To determine if these regional differences in microglial structure and distribution are accompanied by distinctions in functional status, we used imaging, electrophysiology, and transcriptome sequencing to define the basic properties of basal ganglia (BG) microglia and found that microglial anatomical features, lysosome content, and membrane properties differ significantly across BG nuclei. Transcriptome sequencing revealed regional differences in gene expression, including prominent distinctions between midbrain and cortical microglia. Together, these observations demonstrate that microglia in the healthy brain exist along a spectrum of distinct functional states and these data provide a critical foundation for defining microglial contributions to BG circuit function. We are now using similar approaches to define how BG microglia are impacted by chronic drug exposure and the process of normal aging. We are performing whole transcriptome RNAseq of microglia isolated from the NAc, PFC, and VTA of mice following chronic self-administration of saline or cocaine. All tissue is collected for this analysis and sequencing and bioinformatics analysis is ongoing. We are also using imaging, RT-PCR analysis of gene expression, and microglial ablation to quantify aging-induced changes in BG microglia and analyze contributions of these cells to aging-induced alterations in cognitive function.